Ground-relay transmission-line protection



July 18, 1950 w. K. SONNEMANN 2,516,025

GROUND-RELAY TRANSMISSION-LINE PROTECTION I Filed March 2'7, 1947 6 5 l li n2 L5? f-Fl WITNESSES: I INVENTQR William K. Sonnemann.

Patented July 18, 1950 UNITED STATES PATENT OFFICE GROUND-RELAY TRANSMIS SIGN-LINE PROTECTION vania Application March 27, 1947, Serial No. 737,664

4 Claims, 1

My present invention relates to an improved ground-fault protective-relay apparatus for the directional detection of ground-faults in a polyphase line.

Hcretofore, there have been two principal types of directionally responsive ground-fault relays.

One of these previously known ground-fault protective-relay systems has utilized an overcur rent ground-fault detector, which is responsive to the zero-phase-sequence component of the linecurrent, and a ground-directional element which compares the direction of the zero-sequence linecurrent with the zero-sequence line-voltage component. lhis directionally responsive groundfault relaying-system has the advantage of being extremely sensitive and simple, and has enjoyed a long period of successful use in practice, subject only to two disadvantages: (1) it has sometimes been too sensitive, in systems in which zerosequence currents are induced, in the protected line, by faults on a nearby line; and (2) it has been subject to inapplicability or failure, on systems which are subject to those relatively rare fault-conditions which are sometimes referred to as simultaneous ground-faults, in which two ground-faults are in existence at the same time, on two different phase-conductors of two different lines which are connected to a common bus, this defect being limited to transmission systems which are only lightly grounded, through a fairly large grounding-impedance, which very greatly limits the magnitude of ground-faults, except in the case of the aforesaid simultaneous grounds.

The other one of the principal previously known ground-fault protective-relay systems has utilized a zero-sequence ground-fault detector, as, in the first-mentioned system, but a directional element which is responsive to the negativesequence line-current component and the negative-sequence line-voltage component, as described and claimed in the Evans Patent 2,066,932, granted January 5, 1937. This second ground-fault relaying-system has operated satisfactorily, from the standpoint of non-operation on induced current, but it has been subject to the previously mentioned diiliculty in respect to simultaneous ground-faults.

There have also been other, more complicated, ground-fault relays, using diiierent combinations of a plurality of phase-sequence components in both the fault-detecting element and the directional element of the ground-fault protectivesystem.

The principal objects of the present invention areto provide an improved, simple ground-fault relaying-system, which operates satisfactorily in systems which are subject to induced zero-sequence currents in a non-faulted protected linesection, and also to provide simple means whereby my new ground-fault protective relaying-system can be prevented from erroneous operation in the event of simultaneous ground-faults on very lightly grounded transmission-systems, that is, systems having a relatively large grounding-impedance at its grounded points, which may be either at the relay-station bus, or at some other bus.

With the foregoing and other objects in view, my invention consists in the systems, circuits, combinations, apparatus, parts, and methods of design and control, which are hereinafter described and claimed, and illustrated in the accompanying drawing, the single figure of which is a simplified diagrammatic view of one terminal of ground-fault protective-relaying apparatus in accordance with my present invention.

In the accompanying drawing, I show one terminal of a protected three-phase transmission line or line-section l, and the corresponding terminal of a second three-phase line-section 2, which is connected to the same three-phase bus 3 as the protected line-section I. The two linesections I and 2 may be either parallel lines, running along the same right-af-way, or they may be lines running in difierent directions, geographically spaced from each other, except for their termination in the common bus 3. By way of illustration, I have indicated a system in which the three-phase bus 3 is connected to a threephase supply-circuit 4 through a bank of powertransformers 5, having delta-connected primarywindings ii, and star-connected secondary-windings I, having a neutral point 8 which is connected to ground through a grounding-impedance Zg. This grounding-impedance Zg may be either a large impedance, for a lightly grounded system, or a small impedance, or even a substantially zero impedance, in the case of heavily or solidly grounded systems.

My ground-relay terminal-equipment is illustrated as being utilized in the control of the tripping of a three-pole circuit-breaker H, which is utilized to connect the protected line-section l to the bus 3 at the relaying station or terminal. The breaker H is provided with an auxiliary make-contact i la, which is open when the breaker is open, and a trip-coil TC.

The essential features of my relaying apparatus, in its simplest form, consist of a sensitive zero-sequence directional element DO, and a sensitive negative-sequence groundiault-detector or overcurrent-element 0C2.

The directional element 30 similar to ground-fault zero-sequence directional elements which have been previously known the art. It has a current-coil i2 which is ener ized in the residual-circuit 130i a bank of star connected line-current transformers M, so as to be energized in response to the zero-sequence line-cur voltage component E0, by any suitable means,

such as the open-delta secondary-windings I! of a bank of star-connected potential-transformerswhich are energized from the bus 3. The ground-directional element DO also has a makecontact [6, which is connected in a trippingcircuit [9, in series with a make-contact 2d of the negative-sequence overcurrent-element 002, so as to energize the trip-coil TC from a directcurrent source or through the auxiliary make-contact i la of the breaker H.

The negative-sequence overcurrent-relay 002 is energized from a negative-sequence network 2!, which segregates, or selectively responds to, the negative-sequence component I2 of the linecurrents which are supplied to the network from the line-current transformers M.

In cases where my new ground-fault relaying-equipment is to be applied to lightly grounded transmission-systems, having a high-value grounding-impedance Zg, at the station-bus 3 at the relaying-terminal, or at some other bus; and in general in all cases where a general-purpose ground-fault relaying-equipment is to be provided, which is capable of being utilized in all grounded systems, whether the grounding-impedance Zg is large or small or zero, I also provide i an insensitive lockout-relay L02, which is energized from either the zero-sequence line-current I or the negative-sequence line-current 12, so as to be responsive to the heavy line-section currents which flow at M in the event of simultaneous grounds. A preferred connection is shown wherein the lockout-relay L02 is energized from the negative-sequence current-component I2, by having its operating-winding connected in series with the operating-winding of the overcurrent-element 0C2.

The lockout-element L02 is less sensitive than the overcurrent-element 002, as diagrammatically indicated by a smaller number of turns in the lockout-coil L02 than in the overcurrent-coil 002. The lockout-relay L02 has a back-contact, or normally closed contact, 23, which is connected in series in the tripping-circuit l9. When the lockout-relay L02 is utilized, it should be capable of opening its back-contact 23 in a smaller time than the time required for the overcurrent-element 002 to close its make-contact 26, whatever that time may be, whether fast or slow. If necessary, in the event that the overcurrent-element 002 is an element of the instantaneous or fastacting type, it may be provided with a light dashpot 2 3 or other means for retarding its contactclosing response, so as to allow time for the previous opening of the overcurrent-relay back-contacts 23, if the overcurrent relay is responding at all to the fault-conditions which may be existing at the time.

In operation, the setting of the negative-sequence lockout-relay L02, (if this relay is used at all), is so high that it will not respond to single ground-faults, but it will respond when there are simultaneous ground-faults on different phases of two different lines which are connected to the station-bus at the relaying point, as indicated,

for example, by the faults FI and F2 in the draw- 7 ing. It will, of course, respond for phase-tophase or two phase-to-ground faults on the protected line, or such faults fed through the line, but this does not matter, as phase-relays (not shown) are provided to operate for these 4 faults. In general, the lockout-element L02 will not be used unless the system is, or may be, grounded through a relatively large groundingimpedance Zg.

The effect of the large grounding-impedance Zg is to greatly reduce the value of the fault-current, in the event of a ground-fault, but in reducing this ground-fault current, it consumes most of the fault-voltage, leaving very little negative-sequence line-voltage, in the event of a single ground-fault. This was one of the drawbacks of the Evans relay-system, which used a negativesequence directional element (which is energized from voltage as well as current), for the directional response of the ground-fault relayingapparatus.

In the event, however, of simultaneous groundfaults, involving different phases of two different lines, such as the lines I and 2, which are connected to the station-bus, the presence of a large grounding-resistance Zg has heretofore caused an erroneous operation of the directional response of the negative-sequence directional element of the Evans system. In such a simultaneous-fault condition, the ground-fault current, which goes to ground from one of the phase-conductors of one of the lines, does not have to return through the large, current-limiting, grounding-resistor Zg, but returns from the ground directly to another simultaneously grounded phase-conductor of another line, thus returning to the station-bus without passing through the grounding-resistance Zg. Under these conditions, the output of the line-current transformers I l, in the protected linesection, no longer gives a true picture of th positive, negative and zero phase-sequence components of the line-current, because the return-current, in another phase, does not come back through the same set of current-transformers. Because of the distorted conditions just described, ground-relays have previously indicated an incorrect direction, under some simultaneousfault conditions, thus resulting in an erroneous tripping-operation.

Since the indicated negative-sequence currentcomponent is relatively large, under simultane- Gus-fault conditions, as compared to the indicated negative sequence current component which is obtained, from the line-current trans-' formers M and the negative-sequence network 21, when there is a single line-to-ground fault on the protected line-section, it is easy to set the lockout-element L02 so that it will respond to the simultaneous ground-fault condition, and not to the single phase-to-ground fault-condition. If

- fault relays (not shown), which are practically always provided, to protect the line-section against double-ground faults.

In operation, the zero-sequence directionalelement D0 will get enough voltage and current to operate, on single phase-to-ground faults,

when it should, whether the system is grounded through a high impedance Zg or not. This has been amply proved by many years of successful operation of this kind of zero-sequence directional-element.

In the operation of my apparatus, the use of the negative-sequence current-component I2, to energize the overcurrent element or faultdetector element 002, avoids the possibility of false operation which was possible in previous ground-fault relays utilizing zero-sequence overcurrent-elements, in cases when zero-sequence current is induced in a sound line-section by ground-faults on a nearby line, such as the line 2.

In the operation of my invention, the negativesequence lockout-element L02, if used, avoids the completion of a tripping-operation in response to simultaneous ground-faults when a large grounding-impedance Zg is used. Such a lockout-relay, responsive to simultaneous grounds, is also useful, in general, in other types of ground-fault relaying-systems, as in the Evans system using a zero-sequence ground-fault detector and a negative-sequence directionally responsive element.

While I have illustrated my invention in its barest essentials, and in a single illustrative form of embodiment, I wish it to be understood that various changes may be made, by way of additions and substitutions of equivalents, without departing from the essential features of the invention. I desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language.

I claim as my invention:

1. A ground-fault protective-relay apparatus for the directional detection of ground-faults in a poly-phase line, comprising a sequence-responsive means for developing a relaying-current having a substantial, selective response to the negative-sequence component of the line-current in the protected line, a fault-detecting relayingmeans for detecting faults on the line, said faultdetecting relaying-means having a relay-operating element, means for energizing said relayoperating element solely from said sequenceresponsive means, a ground-directional relayingmeans for responding to ground-fault currents flowing in a particular direction, means for energizing said ground-directional relaying-means solely with the zero-phase-sequence line-current and line-voltage, and means for performing a relay-circuit operation in response to responded conditions of both the fault-detecting relayingmeans and the ground-directional relayingmeans.

2. A ground-fault protective-relay apparatus for the directional detection of ground-faults in one of a plurality of polyphase lines which are connected to a common polyphase bus at a relaying-station of a transmission system which is grounded through substantial grounding-impedance, said apparatus comprising a sequenceresponsive means for developing a relaying-current having a substantial, selective response to the negative-sequence component of the line-current in the protected line, a sensitive fault-detecting relaying-means for detecting faults on the line, a less sensitive lockout relaying-means for responding to the heavy negative-sequence current which flows when there are simultaneous ground-faults on different phases of two of the lines which are connected to the common bus, means for energizing said fault-detecting relaying means and said lockout relaying-means from said sequence-responsive means, said lockout relaying-means being sufficiently insensitive to fail to respond in the event of a single groundfault, the response of the fault-detectin relayingmeans being slower than the response of the lockout relaying-means under conditions when both of said means respond, 2, ground-directional relaying-means for responding to ground-fault currents flowing in a particular direction, means for energizing said ground-directional relayingmeans with the zero-phase-sequence line-current and line-voltage, and means for performing a relay-circuit operation in response to a responded condition of said fault-detecting relaying-means, a responded condition of said ground-directional relaying-means, and a non-responded condition of said lockout relaying-means.

3. A ground-fault protective-relay apparatus for the directional detection of ground-faults in one of a plurality of polyphase lines which are connected to a common polyphase bus at a relaying-station of a transmission system which is grounded through substantial grounding-impedance, said apparatus comprising negative-sequence-responsive relay-energizing means for selectively responding to a negative-sequence quantity of the protected line, zero-sequenceresponsive relay-energizing means for selectively responding to a zero-sequence quantity of the protected line, a sensitive ground-fault-detecting relaying-means, a, sensitive ground-fault directionally responding relaying-means, means for energizing one of said sensitive ground-fault relaying-means from the negative-sequenceresponsive relay-energizing means, means for energizing the other of said sensitive groundfault relaying-means from the zero-sequenceresponsive relay-energizing means, sensitive lockout relaying-means for responding to the heavy fault-current which flows when there are simultaneous ground-faults on dilferent phases of two of the lines which are connected to the common bus, said lockout relaying-means being sufficiently insensitive to fail to respond in the event of a single ground-fault, the response of the faultdetecting relaying-means being slower than the response of the lookout relaying-means under conditions when both of said means respond, means for energizing said lockout relaying-means from one of said sequence-responsive relay-energizing means, and means for performing a relaycircuit operation in response to a responded condition of said fault-detecting relaying-means, a responded condition of said directionally responsive relaying-means, and a non-responded condition of said lockout relaying-means.

4. The invention as defined in claim 3, characterized by said negative-sequence relay-energizing means having a selective response to the negative-sequence component of the line-current, said zero-sequence relay-energizing means having selective responses to the negative-sequence components of both the line-current and the linevoltage, and said fault-detecting relaying-means being energized from said negative-sequence relay-energizing means.

WILLIAM K. SONNEMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,554,704 Biemanns Sept. 22, 1925 2,066,932 Evans Jan. 5, 1937 2,110,673 McConnell Mar. 8, 1938 2,295,398 Griscom Sept. 8, 1942 2,315,470 Warrington Mar. 30, 1943 2,327,292 Smith Aug. 17, 1943 

